For safety reasons all buildings require emergency exits and the location of these exits is indicated by suitable signage. In addition to this audible alarms are provided to indicate when a fire occurs.
The problem with the above arrangement is that in large buildings such as warehouses, when a fire occurs the presence of smoke can reduce visibility to such a level that exit signs are impossible to see. In such a situation a fire alarm does not indicate which direction a person should proceed in to escape the fire.